I have a grey t-shirt that had oil on it, so I was hand washing it to remove the oil. It left white marks on it. How do I fix without dyeing it?
If I'm understanding correctly, removing the oil stain took some of the original color from the shirt. If that's the case, there isn't really any way to fix it. Even redying the shirt (assuming it's cotton and will even take dye) won't cover the fact that the original dye has the new dye added, but the lighter patches less of that original dye -- they'll still be lighter.
All you can do is permanently assign that shirt for the kind of work that got the oil stain (and if it's cotton, it's effectively impossible to remove all the oil anyway). If it's a favorite, you might try to find another identical one to wear when you don't want the patches to show.
In this case the best solution is probably to bleach all color from the shirt and have a white shirt instead of a grey one with stains.
Go to a grocery store or drug store and buy bleach from the laundry detergent isle (there are other types of bleach that are not suitable to bleach clothing).
I would mix the appropriate amount of bleach with water in a bucket and put the shirt in the bucket for the appropriate time (read the instructions on the packaging). Keep the bucket outside or in a very well ventilated area. Make sure no children or animals have access to the bucket or the package of bleach! Occasionally stir the shirt in the bucket with a stick or pole. Since the bleach will react with the pole as well, use something you don't need anymore or you can dispose of afterwards.
You have created a unique "pinto" version of your golf shirt.
You are out of luck removing a grease stain that changes the appearance of a fabric if the piece has been dried by heat (such as in a clothes dryer) which will "set" the discolouration. At best, you had a very narrow chance of removing the grease by thoroughly dissolving it in the right detergent IF the chosen one did not affect the dye of the shirt too. Different dyes and and dye processes are affected differently by detergents whether they are pre or post mordant or not.
The damage is irreversible.